Mechanism of 1:1 nonstoichiometric short-range ordering in La-doped Pb(Mg_1/3Nb_2/3)O₃ relaxor ferroelectrics

The mechanism of the 1:1 nonstoichiometric short-range ordering in donor-doped Pb(B′_1/3B″_1/3)O₃-type relaxor perovskites was examined using the La₂O₃-doped Pb(Mg_1/3Nb_2/3)O₃ (PMN) system as a typical example. Based on the dielectric analysis and the transmission electron microscopy (TEM) observation, the observed increase in the degree of diffuse phase transition (DPT) in the presence of La₂O₃ was interpreted as the enhancement of the 1:1 nonstoichiometric short-range ordering accompanied with the growth of the negatively charged nanodomains. The mechanism of the defect process responsible for the 1:1 nonstoichiometric short-range ordering was elucidated by analyzing the electrical conductivity of the La-doped PMN system as a function of the partial pressure of oxygen. It was shown that the substitution of La³⁺ ions for Pb²⁺ ions in the A-site sublattice of perovskite PMN produced the positively charged sites with a concomitant generation of electrons for the electronic compensation. This expedites the growth of the nonstoichiometrically ordered nanodomains in a disordered matrix.